class_docstr: A graph consists of connected Blocks, or Heads.
  
  # Arguments
      inputs: A list of input node(s) for the Graph.
      outputs: A list of output node(s) for the Graph.
code: def _add_node(self, input_node):
      if input_node not in self._node_to_id:
          self._node_to_id[input_node] = len(self._node_to_id)
  
imports: import keras_tuner
  from tensorflow import keras
  from tensorflow import nest
  from autokeras import blocks
  from autokeras import keras_layers
  from autokeras import nodes
  from autokeras.engine import head
  from autokeras.engine import serializable
  from autokeras.utils import io_utils
label_desc: Use image node_id 6 for calling the Graph obj's underlying member method code with example usage: obj._add_node(input_node) without return types